"Progressively" scanned television receivers have been proposed wherein the horizontal scan rate is multiplied, i.e., doubled, and each line of video is displayed twice thereby providing a displayed image having reduced visibility of line structure. In one form of a progressively scanned receiver, each line of video is stored in one of two memories. As a first of the memories is being written with the incoming video signal at a standard line rate, the second of the memories is read two-times at twice the standard line rate thereby providing two lines of "speed-up" (time-compressed) video within one standard line interval. The second memory output is applied to a display having a doubled horizontal sweep rate synchronized with read-out of the memory thereby doubling the number of displayed lines of video signal. An example of such a progressively scanned receiver, wherein the added lines of video signal are replicas of the original scan lines, is described in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983 to R. A. Dischert.
In another form of progressive scan system, the "extra" lines for display are obtained by interpolation of adjacent vertical lines of the incoming video signal. This may be done either before or after "speed-up" (i.e., time compressing) of the video signal in the memory. An example of a progressively scanned display system in which the additional scan lines are obtained by interpolation from the original scan lines prior to time compression or video "speed-up" is described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS which issued Aug. 23, 1983. An alternative of providing interpolation subsequent to speed-up of the video signal is described by Yasushi Fujimura et al. in U.S. Pat. No. 4,509,071 which issued Apr. 2, 1985.
The choice of whether to repeat or interpolate video lines for display in a progressive scan receiver or monitor may depend on the nature of the image being displayed. It has been suggested as being desirable to employ the line interpolation method in a progressive scan receiver when display images of conventional broadcast television and to employ the line replication (repeating) method for displaying characters. See, for example, the paper "Development Of A New TV Receiver Digital Television Adaptable To New Media" by Sony Corp., Aug. 15, 1983. One problem with a receiver having such a dual mode progressive scan capability is that it places an additional burden on the user to change the processing each time he changes video sources. For example, if the user is watching a movie and changes to a television text service or computer source, then he is also burdened with the task of changing the display processing to obtain the sharpest display of character or graphic symbols.